The present invention relates to a method and a system for purifying the exhaust gases of a combustion engine of a vehicle (passenger car, light duty vehicle, etc.). In particular it relates to a method and a system for loading or reloading a decomposition unit of a SCR system.
Legislation on vehicle and truck emissions stipulates, amongst other things, a reduction in the release of nitrogen oxides NOx into the atmosphere. One known way to achieve this objective is to use the SCR (Selective Catalytic Reduction) process which enables the reduction of nitrogen oxides by injection of a reducing agent, generally ammonia, into the exhaust line. This ammonia may be obtained by using different techniques.
One known technique is based on the use of an ammonia precursor, for example an aqueous urea solution (eutectic solution of 32.5 wt % urea in water). Generally, such urea solution is stored in a container mounted on the vehicle. The urea solution is injected into the exhaust line, and the gaseous ammonia is derived from the pyrolytic (thermal) decomposition of the injected urea solution. In case of cold start, it is required to be able to operate the SCR system at the end of a predetermined period of time starting from the engine start, this predetermined period of time depending on the ambient temperature. It is generally used a heating device to liquefy the frozen urea solution in freezing conditions. However, even by doing so, it takes a while before enough urea solution is thawed and injected into the exhaust line. On the other hand, in order to avoid deposits in the exhaust pipe, and insure the required chemical reactions aqueous urea solution must not be injected in the exhaust pipe before the exhaust gases have raised the temperature of the exhaust pipe at a sufficient temperature, typically in the 180° C.-200° C. range.